<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    *{
      padding: 0;
      margin: 0;
    }
    body{
      background: #f5f5f5;
    }
    .box{
      display: flex;
      border: 2px solid red;
    }
    .box div{ /*选择 .box的后代元素 div*/
      width: 234px;
      height: 300px;
      background: #fff;
    }
    .one{
      margin-bottom: 15px;
    }
  </style>
</head>
<body>
  <!-- 弹性盒子 -->
  <!-- 内部的子元素自动水平排列 -->
  <div class="box">
    <div class="one">1</div>
    <div>2</div>
  </div>
</body>
</html>